Cysteine is considered a semi-essential amino acid for newborn infants and is routinely added to neonatal total parenteral nutrition (TPN) formulations. However, the stability of cysteine is low in solution, e.g., it can oxidize to form cystine, a disulfide dimer, which has a lower solubility than cysteine. Most crystalline amino acid solutions have pH levels of about 5-7, which would favor the conversion of cysteine to cystine. For these reasons, cysteine has typically been formulated as a separate parenteral nutrition additive and tends to be mixed with other TPN components just prior to use. Thus, preparations of cysteine with improved stability would be beneficial.
Aluminum is a contaminant commonly found in parenteral nutrition additive solutions. Research indicates that patients with impaired kidney function, including neonates, who receive parenteral levels of aluminum at greater than 4 to 5 μg per kg per day accumulate aluminum at levels associated with central nervous system and bone toxicity. In an attempt to limit the risk of aluminum toxicity, the U.S. Food and Drug Administration (FDA) has rules limiting the total aluminum content in TPN formulations. Since some preparations of cysteine can contain up to 5000 μg/L of aluminum, the total daily aluminum limit may be exceeded. Thus, preparations of cysteine with reduced levels of aluminum would also be beneficial.